Bevel gear units are used in vehicles for changing the torque transmission direction in their drivelines.
Especially, in an all wheel drive vehicle (AWD) it is customary that the front wheels are always driving, whereas the rear wheels are driving only when a limited slip coupling transmits torque thereto under the control of vehicle electronics. The limited slip coupling or AWD coupling is normally attached to a final drive, which transmits torque from a longitudinal propeller shaft, through the AWD coupling, and to the rear wheels. The final drive usually contains a differential.
In conjunction with the motor/gearbox unit and the front wheel axles there is also a bevel gear unit or power takeoff unit (PTU) for transmitting torque to the forward end of the propeller shaft.
The final drive and the power takeoff unit—with its pinion and crown wheel in gear engagement—are normally filled with oil to a certain level. The rotation of the crown wheel is used to throw oil to especially the pinion bearings through channels in the respective housing.
Although the AWD coupling is often attached to the final drive, these two units—usually coming from different suppliers—have separate lubrication systems and are sealed against oil leakage between each other.
For lubrication, the AWD coupling may be filled with oil to a certain level, so that the coupling disc package within the coupling housing rotates in the oil, and bearings, seals, and discs thereby are lubricated and cooled by oil splash and mist.
The rotation in oil bath of the participating parts causes considerable energy losses. The lubricating flow depends to a great extent on the rotational speed and has to be adapted to the worst operational case (high torque). Especially when only the front wheels drive and the rear wheels idle, the transmitted torque is neglectable, which means that the lubricating flow is grossly overdimensioned.